Buried service detection

ABSTRACT

Some embodiments of the invention relate to a mobile detection device for an evaluation of a distance value from the device to an occluded AC-current carrying structure according to its emanated magnetic field, like a location of underground services. The device may include multiple detection coils arranged with a spacing with respect to one another and an electronic signal evaluation unit for detecting the structure according to an electrical signal induced in the detection coils by the magnetic field and to evaluate the distance value according to a difference of the electrical signal in-between at least two of the detection coils.

FIELD OF THE INVENTION

The present invention relates generally to a mobile detection device andto a method of calibrating such a device.

BACKGROUND

It is a common task on construction sites to use devices for detectingunderground structures before or while earth-moving. Such structures areoften occurring in form of services for supplying electricity, gas,fuel, water, or communication data, etc. by underground structures.Although the location of most of these services is or at least should bealready known from a surveyor's plan of the site, their locations canhave uncertainties or there could be additional services that are notmentioned therein. Often underground services are also simply overlookedor wrongly assessed by the operator of an earth moving machine duringwork.

Avoidance of damage to underground structures while digging in a trenchor in areas being excavated is an important task. As damage to a servicecan cause serious impact and costs, additional measurements are taken inorder to be able to detect the proximity or preferably the exact depthof such services on the site before or while excavating. Therein, it isnot only of interest to determine the path, which the buried service isfollowing, but also to determine the depth at which the service isburied, or in other words to determine the distance from the detectiondevice to the service. The distance from the device to the service willfurther also be referred to as depth, as a common term used forunderground wirings, conduits or pipes. Devices for this purpose areknown as Cable Detection Tools or Cable Avoidance Tools—also called CAT.An embodiment of such a device is for example described in EP 2 362 241.Such a detection device is mostly embodied movable, which means it canbe designed and built as a handheld device to be carried around by aworker. In special movable embodiment of the detection device, it canfor example also be mounted at a bucket of an excavator and move withthe bucket. In view of this, the detection device is preferably embodiedlightweight and small-sized.

One way to locate underground services is to detect electromagneticfields sent out by the service itself. To do this, the service requireshaving a naturally occurring electrical signal which emits anelectromagnetic field that is detectable above the ground, such as e.g.a live power supply line. To detect other types of services as well, forexample a wiring system of switched off street lights, unused orlow-voltage communication cables, gas- or water-pipes, additionalmethods are known. In U.S. Pat. No. 4,438,401 metallic services withoutnaturally occurring signals are directly connected to asignal-generator. In this way, an electrical signal can be coupled tothe service, and therefore it is possible to detect it by itselectromagnetic field. U.S. Pat. No. 5,194,812 shows a solution fordetecting hollow pipes like gas or water pipes by introducing aconductor or sonde into them—or by laying a conductor next to theservice—that will function as a transmitter for the field to bedetected. In EP 9 166 139 or EP 2 645 133, a field emitting signal iscoupled into a conductive underground structure by introducing a currentfrom an AC current-source into soil by earth-spikes or other groundconnection means, wherein the current follows along the conductivestructure as path of least resistance through soil.

What all the mentioned detection systems have in common, is that theunderground structures need to emit an electromagnetic field that isstrong enough to be detectable above the surface, especially it has tobe detectable non-ambiguously in respect of the always presentnoise-floor of various other electromagnetic fields from other sources.The electromagnetic fields emitted by different services reside indifferent ranges of frequency, dependent on the signals present on theservice. Power-lines commonly provide currents with a fundamentalfrequency of 50 Hz or 60 Hz, dependent on the country, and thereforeemit fields with the same fundamental frequency. But as e.g. describedin EP 2 362 241, also harmonics of the above mentioned frequencies canbe used for mains detection, in particular zero sequence harmonics.

Signals that are artificially applied to the structures (either bydirect or by soil connection) are restricted in frequency byradio-communication-rules which are country-dependent and given e.g. toavoid interferences with radio communication services. A special exampleof frequencies allowed in the UK for a general geographic surveillanceuse, such as cable detection, are the frequencies of 8 kHz or 33 kHz,which are used by some CAT-equipment. For example, the VLF radio bandrange (=Very Low Frequency radio waves e.g. in the range of about 15 kHzto 60 kHz), especially the low wavelengths in the range of myriameter,are known to penetrate soil material quite well and can therefore beused for cable detection purpose.

The fields emitted by communication lines are another importantdetection target. For those services, no special single frequency can beexpected but rather a range of frequencies has to be taken into account.Still, the emission of frequencies in those bands which are lessstrictly regulated will likely dominate.

For example as shown in WO 2011/104314, WO 2008/064851 or WO2008/064852, the depth or distance to a buried service, which can beconsidered as a long current carrying conductor, can be determinedaccording to the signal strength difference at two pickups located in aknown spacing to each other.

A problem therein is that such a depth determination—which depth can bea rather critical value for excavation tasks—is quite sensitive totolerances in the device's components and manufacturing process, inparticular to the characteristics and arrangement of the detectioncoils.

Therefor, EP 1 843 177 describes a factory calibration rig, in which anindividual fine tuning of each cable detection device can be determinedin a factory or laboratory environment, in particular after device'sfabrication or later on at a certification authority.

Once calibrated, the detection device is regularly exposed to quiteharsh environmental conditions at worksites, heat concussions andvibrations in cars when transported, accidental dropping or knockingover, exposure to direct sunlight, snow, rain, water, dirt, etc.Therefore, the factory calibration data might be ill fitting in fieldoperation. In particular, aging and temperature drifts of theelectronics and a displacement of the coils in field-usage can havenegative impact. Therefore, the guarantied accuracy levels of the depthvalues determined by such devices is in general relatively low, e.g.within some decimetres to metres.

SUMMARY

Some embodiments may improve such a cable detection device, inparticular to improve the accuracy and/or reliability of its depthdetermination.

Some embodiments may improve the robustness of the detection deviceagainst environmental influences and electrical and mechanicaltolerances.

Some embodiments may provide a detection device for buried services,which is built to be self-calibrated, without requiring externalequipment or a special setup, in particular field calibrated, so thecalibration can be done or verified by the device on its own, at anytime and location.

Some embodiments may provide a detection device which can be resized, inparticular to require less storage space while still providing highdepth detection accuracies.

According to some embodiments of the present invention, a device withimproved calibration capabilities is provided, in particular byproviding a device which is built to be field calibrated by acorresponding calibration method, which can preferably be executedanytime and anywhere by an inexperienced operator, e.g. in field orbefore each detection usage. The calibration should preferably be donequickly and should be robust against external influences.

Some embodiments of the present invention, therefore, relates to amobile detection device for an evaluation of a distance value from thedevice to an occluded AC-current carrying structure according to itsemanated magnetic field. For example, a cable detection device for alocation of buried services like electrical wiring, cables, gas or waterpipes, etc. which are occluded underground at a construction site. Thedevice is thereby in particular built to determine the depth, in whichthe service is buried.

The device comprises at least a first and a second coil, each comprisingat least one winding of an electrically conductive material. The windingis enclosing an area and can be of arbitrary shape, e.g. round,circular, rectangular and preferably substantially flat, with a heightcomparably much lower than the outer dimensions of the enclosed area.

The coils are arranged at the detection device with a fixed locationwith respect to one another. The locations can be defined by a spacingin-between the coils, for example by a spacing of the coil axis, whichcan be defined in the centre of and normal to the coil area. Thelocations are fixed, which means that the coils are not built to bemoved with respect to each other during a depth measurement.Nevertheless, e.g. due mechanical shocks, temperature influences or thelike, the coils locations can vary in small tolerances over time.

In a special embodiment, the device can be embodied to be foldable,collapsible or telescopic, for example to reduce the device's size forstorage and/or transportation into a first position and to expand thedevice into a second position for usage, in which the spacing in-betweenthe coils is increased for good detection results, wherein the secondposition can be fixed. In such an embodiment, the below discussedself-calibration according to the present invention can be especiallyadvantageous, as the exact location of the fixing in the second positionmay slightly vary from expansion to expansion, and such a variation canbe compensated by a calibration after each expansion. The presentinvention therefore also relates to a mobile cable detection device,being mechanically resizable in at least two fixable positions, inparticular foldable, collapsible or telescopic, in which positions adistance in-between the coils for detection is different, which devicecan in particular comprise a calibration as discussed herein.

The device also comprises an electronic signal evaluation unit fordetecting the structure. The detection is done according to electricalsignals, which are induced in the coils by the magnetic field emanatedby the structure. The signal evaluation unit therein evaluates thedistance value from the device to the structure according to adifference of the electrical signal from at least two of the detectioncoils.

In particular, the signal evaluation unit can thereby comprise at leastone of:

-   -   an amplifier circuit connected to the coil, for amplifying the        electrical signal induced in the coil by the magnetic field from        the structure,    -   a bandwidth limiting filter for the electrical signal, which is        built to suppress undesired frequency ranges and/or to avoid        aliasing,    -   an analog to digital converter for digitizing the filtered        output of the amplifier circuit to a time and value discrete        digital representation, and/or    -   a computation unit built in such a way to detect the structure        according to an evaluation of the digital representation, in        particular according to a difference in signal strength        in-between the at least first and second coil.

In particular, no information content is contained in the electricalsignal from the detection coil or if, such information content is notevaluated by the detection device for the purpose of extracting theinformation content itself, at most, a possible identification of thesignal as such is done by the detection device.

According to the present invention, the detection device comprises acalibration unit. This calibration unit is built to successivelyconfigure one of the at least first or second coil of the device astransmitter for a defined electrical excitation signal to emit amagnetic calibration field, during a calibration routine. The electricalexcitation signal can in particular be defined in one or more offrequency, current strength, and/or modulation.

The magnetic calibration field is received or detected by at least oneremaining of the at least first or second coil, which is at present notconfigured as transmitter.

Calibration parameters for the detection device are determined by thecalibration unit based on the received calibration field, in particularcalibration parameters for the depth determination, like an offsetparameter and/or a scaling parameter for at least the first and thesecond coil. Another calibration parameter could e.g. be a phase shiftparameter and/or coil location parameter.

The thereby determined calibration parameters can then be applied to theelectrical signals from a to be detected structure (in their digitaland/or analog representation), for an at least partial compensation ofdiffering receiving characteristics of the at least first and secondcoils and/or their corresponding signal evaluation paths of the signalevaluation unit, in particular of the amplifier, filer and/or analog todigital converter.

The electrical excitation signal can be generated by a signal generator.In one embodiment, there can be one signal generator selectivelyconnectable to one of the coils by a switching means, so that thecalibration unit is built to apply the same excitation signal to apermutation or succession of the at least first and second coil. Inanother embodiment, there can be a dedicated excitation signal generatorfor each of the detection coils.

In an embodiment, the electrical excitation signal can in particularhave known electrical characteristics, which can be defined by design ofthe signal generator or measured.

For example in a first embodiment of this aspect, the excitation signalcan be measured by the receiving section of the electronic signalevaluation unit which is normally used for detecting the structure, whenthis receiving section is connected in parallel to the electricalexcitation signal that is applied to the one of the detection coilswhich is transmitting calibration field. In a second embodiment of thisaspect, the coil can be switched electively to either the receivingsection or to the generating excitation signal source, wherein theexcitation signal is either defined by design of the source (e.g. fixedby design or regulated) or measured by other means. In a thirdembodiment of this aspect, the excitation signal can be applied withoutexactly defined or measured knowledge of its electrical characteristics,which can—not only—but in particular be used, if the same excitationsignal is electively switched to one of the detection coils.

In an embodiment, in particular with more than two coils, the electricalexcitation signal is not required to have exactly known electricalcharacteristics, as discussed further below, although the frequency ofthe excitation signal should preferably be within the detectionbandwidth of the device or coarsely correspond to the frequency of thefields from the structures to be detected.

In other words, in the present invention, the at least first and secondcoil of the detection device is operable as a transmitter for a magneticcalibration field, which calibration field is picked up by the remainingof the coils and the thereby resulting values are evaluated to determinecalibration parameters for the coils. Therefore, the device according tothe present invention is capable of a self-calibration without arequirement for external calibration devices or an application ofdefined and known magnetic fields from external to the device.

As said, the detection device comprises a calibration unit built toconfigure one of the detection coils as transmitter for a calibrationfield by applying an electrical excitation signal, which calibrationfield is detected by the remaining of the detection coils and thedetected values of the calibration field, in particular its strength arestored. Thereon, the calibration unit determines calibration parametersfor the detection coils based on stored values of the detectedcalibration field.

In a special example of an embodiment, the detection device can compriseat least a first and a second of the detection coils, in particularexactly two—the first and the second detection coil, and the calibrationunit is built to successively configure one of those detection coilsafter the other as transmitter for the calibration field, whiledetecting the calibration field with the remaining detection coil anddetermining coil-calibration parameters there from.

In another special example of an embodiment, the detection devicecomprises at least a first, a second and a third detection coil, inparticular exactly three—the first, the second and the third detectioncoil, and the calibration unit is built to successively apply theelectrical excitation signal to one of the at least first, second orthird detection coil after the other in order to configure it totransmit the calibration field. The calibration field is then detectedby the two remaining of the detection coils, to which the excitationsignal is not applied, and a set of the detected values of thecalibration field, in particular its strength values, are stored.Thereupon, the calibration unit determines calibration parameters for atleast two of the coils, based on the stored calibration field values.Therein, the calibration parameters can for example comprise a gain andan offset parameter for field values determined by the detection coils.Optionally, also the spacing in-between the detection coils can bedetermined as a calibration parameter, which can then be used tocompensate for errors during a detection of a structure.

The invention can also be implemented in device with more than threedetection coils. Preferably, the calibration unit determines calibrationparameters for each detection coil (which determination considers by itsnature also the coils evaluation circuit).

The electrical excitation signal can be generated by a current signalgenerator (which can have a regulated output signal). In an embodiment,there can be a single signal generator whose output is selectivelyswitchable to one of the detection coils, so that all detection coilscan be excited substantially by an equal signal. The excitation signalcan for example be a sine wave of known frequency. In anotherembodiment, the excitation signal can also have another waveform and/orcan have a variable or modulable amplitude and/or frequency, whereby ina special example the calibration signal can be modulated or coded insuch a way to make it uniquely distinguishable from other environmentsignals and noise.

As a certain aspect of the invention, which could also be considered onits own during a detection of structures, the electronic signalevaluation unit can comprise an amplifier circuit embodied as a currentsensing amplifier as a first amplification stage which is connected tothe detection coil. It can in particular be embodied as a transimpedanceamplifier, preferably having a low input impedance, for example below100 Ohm in the relevant frequency range of about 50 to 100.000 Hz, andcan be tuned to have a substantially linear output over a frequencyrange of about at least 100 to several hundred kHz.

In one practical embodiment, the current sensing amplifier can comprisean operational amplifier (OpAmp) with a feedback network in a current tovoltage configuration. This circuit can in particular be embodied in waythat the negative OpAmp input is connected to one end of the coil, theother end of the detection coil is connected to the positive OpAmp inputand the OpAmp output is fed back to the negative OpAmp input by a firstimpedance, in particular a resistive and capacitive impedance.

In another practical embodiment, the current sensing amplifier can beembodied with an input stage comprising a JFET in a feedback loop of afirst operational amplifier stage in a current to voltage configuration.Therein, the JFETs-Gate can for example be connected to the detectioncoil and with the feedback network. Preferably, this circuit alsocomprises an active operation point setting and/or bootstrappingcomprising a second OpAmp that is biasing the first operationalamplifier stage.

Therein, the detection coils each can have a low winding count of 1 to500 turns of a conductor with a cross-section of at least about 0.1 mm²,wherein the winding of the detection coil encloses an area of more than100 cm² and below 0.5 m², preferably with a approximately rectangular orcircular cross-section. In any of above embodiments, the outputimpedance of the detection coil can be above the input impedance of thefirst amplification stage to which the detection coil is connected.

In an embodiment of the invention, the detection coils are embodied astracks on a PCB, wherein the PCB also comprises the current sensingamplifier mentioned above.

In a regular embodiment according to the invention, the detection coilhas a single multiturn-winding with two ports, which will be used aswell for transmitter as for receiving magnetic fields. In a specialembodiment, the detection coil can alternatively comprise a dedicatedtransmitter winding portion for application of the excitation signal anda dedicated receiver winding portion for detecting magnetic fields,preferably wherein both winding portions are magnetically coaxial toeach other, and wherein the two portions are on a common rigid,preferably one-pieced, carrier.

The invention also relates to a calibration method for a mobiledetection device which comprises multiple detection coils for anevaluation of a distance or depth value from the detection device to anoccluded AC-current carrying structure, in particular for a location ofservices occluded underground at a construction site, according to thestructures emanated magnetic field. This calibration involves, applyingan electrical excitation signal to one of the detection coils, whichthen acts as a magnetic field transmitter for a calibration field, andreceiving and evaluating the calibration field by the remainingdetection coils. Transmitting and receiving is done multiple times witha permutation of the detection coils usage for transmitting orreceiving, preferably for all possible permutations. Then a determiningof calibration parameters, in particular an offset and gain calibrationparameter, for at least two of the detection coils, preferably for allcoils, is done by a calibration unit, based on a set of the evaluatedcalibration fields from each of the permutations.

Therein, the detection device can comprise at least a first, a secondand a third detection coil, in particular only a first, a second and athird detection coil. and the calibration unit is successively applyingan electrical excitation signal to one of the at least first, second orthird detection coils for transmitting the calibration field. In eachsuccession, a detecting of the calibration field is done by theremaining of the detection coils (to which the excitation signal ispresently not applied) and storing of the values of the calibrationfield, in particular of a field strength value. A determining of thecalibration parameters is then done for each of the detection coils,based on the stored values.

Therein, the receiving and evaluating with the detection coils cancomprise an amplifying of the detection coils output in current mode bya current mode amplifier, which is providing low input impedance to thedetection coil, in particular with a winding count of the detection coilbelow 100.

The method, or at least those parts of it which involve computationand/or calculation, can also be embodied as a computer program productthat is stored on a machine readable medium or embodied aselectromagnetic wave (such as e.g. a wired or wireless data signal).Consequently, the invention further relates to such a computer programproduct comprising program code for a calibration of a detection deviceaccording to the invention. The program code is therein in particularbuilt for executing

-   -   an applying of an excitation signal to one detection coil of the        detection device for emanating a calibration field,    -   a receiving of the calibration field by at least one other coil        of the detection device and storing a field value, wherein the        applying and receiving is done alternately for the detection        coils of the detection device, preferably to cover all possible        permutations, and    -   a calculating of calibration parameters for the detection device        based on the stored field values.

The computer program can be executed in a calibration unit of adetection device according to the invention, which device therefore alsoinvolves a computation means built to run a computer program providingcalibration functionality according to the invention, with or withoutthe computer program actually loaded.

BRIEF DESCRIPTION OF THE DRAWINGS

Devices, methods and setups according to the invention are described orexplained in more detail below, purely by way of example, with referenceto working examples shown schematically in the drawing. Specifically,

FIG. 1 shows an example of an embodiment of a mobile detection device towhich the present document relates;

FIG. 2a and FIG. 2b are showing an example of a first embodiment of adetection device and its calibration according to the invention;

FIG. 3a , FIG. 3b and FIG. 3c are showing example of a second embodimentof a detection device and its calibration according to the invention;

FIG. 4a , FIG. 4b and FIG. 4c are showing examples of embodiments of adetection device according to the present invention which are resizable,in particular with a calibration according to the invention;

FIG. 5 shows an example of a block diagram illustrating the principle ofa self-calibration of a detection device according to the invention;

FIG. 6 shows an example of a known coil detection circuitry;

FIG. 7 shows an example of a baud plot of a circuitry of FIG. 6;

FIG. 8 shows an example of a noise plot of a circuitry of FIG. 6;

FIG. 9 shows an example of a first embodiment of a circuit according toa certain aspect of the invention;

FIG. 10 shows an example of a second embodiment of a circuit accordingto the certain aspect of the invention;

FIG. 11 shows an example of a third embodiment of a circuit according tothe certain aspect of the invention;

FIG. 12 shows an example of a bode plot of a circuit according to thecertain aspect of the invention, like the one of FIG. 10;

FIG. 13 shows an example of a noise plot of a circuit according to thecertain aspect of the invention, like the one of FIG. 10;

FIG. 14 shows a comparison of sensitivity plots of the known coildetection circuitry and the ones according to the certain aspect of theinvention;

FIG. 15 shows a possible embodiment of a detection device according tothe invention comprising the certain aspect.

The diagrams of the figures should not be considered as being drawn toscale. Where appropriate, the same reference signs are used for the samefeatures or for features with similar functionalities. Different indicesto reference signs are used to differentiate between differentembodiments of a feature which are shown as examples.

DETAILED DESCRIPTION

FIG. 1 shows an embodiment of a mobile detection device 1 for anevaluation of an “a” distance value 13 from the device 1 to a structure10 buried in ground 12, which is carrying “i” as an alternatingelectrical current 11 and which thereby emanates a magnetic fieldBiB,BiA. The device 1 comprises at least two coils, as the shown“A”-coil 2 a and a “B”-coil 2 b, which are arranged with “d” as spacing3 with respect to each other. The magnetic field emanated from thecurrent 11 in the structure 10 is illustrated by the magnetic fieldlines BiA and BiB, where the first character B indicates a magneticfield, the second character “i” indicates the source of the field—whichis the current 11, and the third character indicates the place ofmeasurement—which is the “A”-coil 2 a respectively the “B”-coil 2 b.

The detection of the structure or service 10, which can e.g. be anunderground cable, sonde, pipe, etc. carrying per se or artificiallyintroduced an electrical current, is done by detecting the magneticfields BiA,BiB by the coil 2 a and the coil 2 b. The buried service 10can therein be considered as a long current carrying conductor, emittingthe magnetic field. The depth 13 from a defined point of the device 1 tothe buried service 10 can be mathematically expressed by the formula

${{Depth} = \frac{d}{\left( \frac{A}{B} \right) - 1}},$

wherein “Depth” is the value “a” of the depth 13, “A” is the magneticfield strengths at the coil 2 a and “B” at the coil 2 b, and “d” is theseparation 3 of coil 2 a and coil 2 b.

Strictly speaking, the depth 13 is in general determined with respect tothe central axis of the coils 2 a,2 b comprised in the device 1, and notwith respect to ground level. This can e.g. be overcome by defining thedetermined distance which is determined with respect to the underside ofthe device 1 (e.g. by an according offset of the depth value provided bythe device 1) and putting down the underside of the device 1 on groundlevel for an exact measurement. Another option is to define a workingdistance from the underside to the ground 12, at which the device 1 hasto be operated and providing the depth value 13 with respect to thethereby resulting offset. Yet another, probably novel approach for suchdevices 1 and an invention on its own is, that the detection device 1comprises at least one proximity-sensor, in particular an optical orultrasound distance measurement unit, at the underside, for measuring aspacing from the device's underside to ground level, preferably in thesame measurement direction as the magnetic depth determination. Theactual ground-spacing-value determined by the proximity-sensor is thenused as an offset for the determined depth 13, so that the indicateddepth value is always relative to ground level, regardless how thedevice 1 is held.

Apparently, also more then the above described at least two coils 2 aand 2 b can be used for the detection, in particular for providingredundant data and/or increased accuracy. For the detection of thestructure 10 and its depth 13 determination, the device 1 comprises a(here not explicitly illustrated) electronic signal evaluation unit.Preferably, the evaluation unit comprises a programmable and/orhardwired digital computation unit, like a microprocessor, DSP, FPGA,ASIC, etc.

The above described detection relates to the signal strength of themagnetic fields BiA,BiB which are determined by means of the coils 2 aand 2 b as well as on the location of the coils 2 a and 2 b with respectto each other. In view of tolerances and aging of the components andtheir locations in the detection device 1, the introductorily mentionedprior art uses an external calibration rig with Helmholtz-Coils togenerate electromagnetic fields that emulate those of a long straightcable in defined distance, to which the detection device 1 is thenfactory-calibrated.

FIG. 2a and FIG. 2b are illustrating an embodiment of the principle ofthe approach according to the present invention, according to which aself-calibration of the detection device 1, for example also in thefield—without requiring an external calibration rig, is achieved. Such aself-calibration according to the present invention, which e.g. can beembodied by a calibration unit providing a special calibration mode orfunction of the device 1, is explained in the following. The here notexplicitly shown calibration unit of the detection device 1 can therebye.g. also at least partially use the same hardware resources as theevaluation unit.

In the arrangement shown in FIG. 2a , the coil 2 a is configured inordinary receiving mode, which is equal to the one used for the abovedescribed detection of a buried service 10, e.g. the coil 2 a beingconnected to an amplifier, filter and/or analog to digital converter.

The role of the coil 2 b is reversed, as it is not used for reception asduring detection, but used to transmit a known signal, resulting in amagnetic field Bt. This can be achieved by applying an electricalexcitation signal to the coil 2 b, which signal preferably has a knowncharacteristic, such as frequency, current strength and/or phase. Toachieve such, coil 2 b can be connected or switched to an electricalsignal source, in particular a current source, by the calibration unit.The excitation signal is an ac signal, preferably within a frequencyrange of the detection device 1 that is also used for the detection ofthe structure 10. For example either with a frequency of mains supply(like e.g. 50 or 60 Hz) or one of its harmonics, with a standarddetection frequency provided by a detection signal injection source thatcan be applied to a service 10 (like about 8 or 33 kHz) and/or within aradio range of e.g. about 15 kHz to 60 kHz.

The coil 2 a now receives the magnetic field Ab emitted by the nowtransmitting coil 2 b and the detected signal can be analyzed and/orstored by the calibration unit, e.g. the signal strength and/or aneventually occurring phase shift can be determined.

By the receiving circuitry and electronics, a certain received signal ofinterest can be selected and the system will be hardly influenced by anyoutside interference, or noise, wherefore specific frequencies fortransmission and reception can be selected, and/or the excitation signalcan be specifically coded or modulated and the reception signalaccordingly demodulated to differentiate the calibration signal fromother external signals (still, no information content of the calibrationsignal is evaluated and provided to another means). According to theinvention, not only the characteristics of the receiving coil 2 a, butalso those of the whole receiving circuitry up to the digitalized valuescan be determined and calibrated.

As shown in FIG. 2b , in the present invention, the functionalities canbe reversed, and the coil 2 a can be configured as the transmitter forthe field At and the coil 2 b as the receiver for the field Ba from coil2 a. The determined field Ba can also be evaluated and/or stored. In aspecial embodiment, the same excitation signal, preferably from the sameexcitation signal source, can be applied successively to coil 2 a andcoil 2 b, for example by an according switching unit.

Based on the stored data with respect to the received calibration fieldsAb,Ba, the calibration unit can self calibrate the detection device 1 bydetermining calibration parameters for the coils 2 a,2 b, by whichparameters deviations in offsets and/or gains can be adjusted to zero.Those calibration parameters can then be applied to the field signals ofthe structure 10 to be detected during a detection, in order tocompensate inaccuracies and to gain good detection results and a correctdepth value 13.

In an illustrative example of a practical embodiment in FIG. 2a and FIG.2b , two coils 2 a A and 2 b B (often also named “Top”- and“Bottom”—antennas in cable locator devices) are separated by a distance3 “d”. It can be assumed that the two coils 2 a,2 b are substantially inthe same plane and any offsets in the vertical plane are negligible.According to the present invention each of the coils 2 a,2 b can also beconfigured as a transmitter; an excitation signal applied to coil 2 aconfigures it as a transmitter for a calibration field At which fieldcan be received by coil 2 b and vice versa. The excitation signal canfor example be a pure sine wave of a known frequency, chosen.

In practical designs it can happen that, the coil spacing 3 “d” caninitially not be accurately controlled due to the manufacturingprocesses and/or “d” may vary with temperature and time. A small changein “d” can give a significant change in the received signalstrength—wherefore the depth determination will get inaccurate.

As shown in FIG. 2b , the magnetic field Ba at a distance 3 from thetransmitter coil 2 a that has a coil radius R, is given by the BiotSavart Law, known from textbooks:

$B = \frac{\mu_{0}\; R^{2}}{4\pi \; 2\left( {R^{2} + d^{3}} \right)^{3/2}}$

To simplify the analysis it is assumed that Area=the cross sectionalarea of the coil, so that

${\left. B \right.\sim\frac{\mu_{0}\; {Area}}{2\pi \; d^{3}}}.$

In conformance with both FIG. 2a and FIG. 2b , the following terminologywill be used:

-   -   Ba=received signal at B from A,    -   Ab=received signal at A from B,    -   At=transmitted signal from A,    -   Bt=transmitted signal from B.

According to the formula, the received signal is proportional to (1/d)̂3(which ratio is the same for a field from a structure during detection),which gives:

${\left. {At} \right.\sim\left( \frac{Ba}{d} \right)^{3}},{{and}\mspace{14mu} {{\left. {Bt} \right.\sim\left( \frac{Ab}{d} \right)^{3}}.}}$

If everything would be perfect, Ba and Ab would be equal. As this is notthe practical case, a calibration value Ks to compensate for anyvariation in “gain” is introduced. So a calibration of the detectiondevice 1 is achieved by determining a calibration parameter, which makessensor 2 b give the same readings as sensor 2 a, such that:

Ab=Ba·Ks.

If it can be assumed in a certain embodiment that the locations of thecoils 2 a,2 b with respect to each other can be considered fixed and/orknown, their spacing 3 can also be included in the calculation of thecalibration parameters. For example, a value for the spacing “d” of 0.5m is theoretically giving a known field-ratio of 8. Therefore, using thecoil 2 a as a transmitter, we expect a signal ratio in-between coil 2 aand coil 2 b of 8, and based on the actually measured values,calibration parameters for a correction for anomalies in the ratiomeasurement can be determined. This can be repeated for multiple or allcoils. The nature of the electromagnetic detection coils 2 a,2 b, forthis application can be considered reversible; in that when a coil isconfigured as electromagnetic transmitter, it can be regarded to havethe inherent capability to transmit with substantially equalcharacteristics as it receives. If the same results are obtained, thenit can be concluded that there are no differences in the response and/orsensitivity of the two coils 2 a and 2 b.

The excitation signal can therein be sequentially applied to each one ofthe coils 2 a,2 b after the other, while the remaining coils areconfigured to detect the thereby induced calibration signal. It is notrequired to excite multiple coils with coordinated excitation signals asin the external Helmholz calibration rig of prior art, but optionally,in an embodiment with an additional step, the calibration could beverified by applying a defined difference in the excitation current toeach of the transmitting detection coils to verify the above analysis.

In a special embodiment, the calibration can also be repeated formultiple excitation signal frequencies to cover a possible nonlinearfrequency response of the coils 2 a,2 b and/or the receivingelectronics, for example once in the mains frequency range and once inthe radio frequency range. The coils 2 a,2 b can each on its own beevaluated by a detection circuit. For the present invention, it is notrequired to connect the coils 2 a,2 b into a serial or parallelconnection.

FIG. 3a , FIG. 3b and FIG. 3c , are showing an example of an embodimentof the invention regarding a calibration for a mobile cable detectiondevice 1, which comprises three coils A°2 a, B°2 b and C.°2 c.

The three coils 2 a, 2 b and 2 c are separated by the distances “d1” 3 aand “d2”°3 b in a Cable Location Instrument 1. This arrangement of thecoils 2 a, 2 b and 2 c is also used for the location of occludedstructures 10.

As discussed above, according to the present invention, a calibrationunit is built in such a way that the coils 2 a,2 b,2 c can be configuredas transmitter for calibration fields At,bt,Ct. Thereby, e.g. coil 2 acan receive a signal transmitted from coil 2 b (FIG. 3b ) or from coil 2c (FIG. 3c ); Coli 2 b can detect signals from coil 2 a (FIG. 3a ) orcoil 2 c (FIG. 3c ), and so on . . . .

In the following, the terminology will comprise:

-   -   At=transmitted signal from coil A;    -   Bt=transmitted signal from coil B;    -   Ct=transmitted signal from coil C;    -   Ab=received signal at coil A from coil B;    -   Ac=received signal at coil A from coil C;    -   Ba=received signal at coil B from coil A;    -   Bc=received signal at coil B from coil C;    -   Ca=received signal at coil C from coil A;    -   Cb=received signal at coil C from coil B;

Consider FIG. 3a , where coil A 2 a is configured as transmitter of thefield At, which is received by coil B 2 b as field Ba and by coil ° 2 cas field Ca.

In general, the magnetic field B at a distance d from the transmitter,of a coil radius R, is again given by the Biot Savart Law:

$B = \frac{\mu_{0}\; R^{2}}{4\pi \; 2\left( {R^{2} + d^{3}} \right)^{3/2}}$

Again, with the simplifying assumption that: Area=the cross sectionalarea of the coil, which results:

${\left. B \right.\sim\frac{\mu_{0}\; {Area}}{2\pi \; d^{3}}}.$

When coil A is applied an excitation signal, transmitting the field Atwhereof Ba and Ca are the measured values of the received fields, thiscan be written as:

$\begin{matrix}{{{At} = {\frac{Ba}{\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {{\frac{Ca}{\left( {d\; 1} \right)^{3}}{At}} = {\frac{Ba}{\left( {{d\; 1} + {d\; 2}} \right)^{3}} = \frac{Ca}{d\; 1^{3}}}}}},} & (1)\end{matrix}$

and similarly for FIG. 3b and FIG. 3c :

$\begin{matrix}{{Bt} = {\frac{Ab}{\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {{\frac{Cb}{d\; 2^{3}}{Bt}} = {\frac{Ab}{\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {\frac{Cb}{d\; 2^{3}}\mspace{14mu} {and}}}}}} & (2) \\{{Ct} = {\frac{Bc}{\left( {d\; 2} \right)^{3}} = {{\frac{Ac}{\left( {d\; 1} \right)^{3}}{Ct}} = {\frac{Bc}{d\; 2^{3}} = {\frac{Ac}{d\; 1^{3}}.}}}}} & (3)\end{matrix}$

This gives:

$\begin{matrix}{{{{From}\mspace{14mu} (3)\frac{d\; 1^{3}}{d\; 2^{3}}} = {{\frac{Ac}{Bc}\frac{d\; 1^{3}}{d\; 2^{3}}} = \frac{Ac}{Bc}}};} & (4) \\{{{{From}\mspace{14mu} (2)\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {{\frac{d\; 2^{3}{Ab}}{Cb}\left( {{d\; 1} + {d\; 2}} \right)^{3}} = \frac{d\; 2^{3}{Ab}}{Cb}}};} & (5) \\{{{{{From}\mspace{14mu} (1)\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {{\frac{d\; 1^{3}{Ba}}{Ca}\left( {{d\; 1} + {d\; 2}} \right)^{3}} = {{{\frac{d\; 1^{3}{Ba}}{Ca}.{Therefore}}\text{:}\mspace{14mu} \frac{d\; 2^{3}{Ab}}{Cb}} = \frac{d\; 1^{3}{Ba}}{Ca}}}},{and}}{{\frac{d\; 1^{3}}{d\; 2^{3}} = {\frac{{Ab} \cdot {Ca}}{{Ba} \cdot {Cb}} = \frac{Ac}{Bc}}},{{resulting}\mspace{14mu} {in}}}} & (6) \\{\frac{{Ab} \cdot {Ca} \cdot {Bc}}{{Ac} \cdot {Ba} \cdot {Cb}} = {{1\; \frac{{Ab} \cdot {Ca} \cdot {Bc}}{{Ac} \cdot {Ba} \cdot {Cb}}} = 1.}} & (7)\end{matrix}$

This result is independent of the distances, d1 and d2.

To calibrate the sensors, a modification of the measurements detected bythe coil 2 b relative to coil 2 a can be done by calibration parameters:

A=Ks·B+Ko,

where Ks is a calibration scaling factor and Ko is a calibration offset.

Substituting for Ab in (7) results

$\begin{matrix}{{\frac{\left( {{{Ks} \cdot {Bb}} + {Ko}} \right)}{Ac} \cdot \frac{Ca}{Ba} \cdot \frac{Bc}{Cb}} = {{1\; {\frac{\left( {{{Ks} \cdot {Bb}} + {Ko}} \right)}{Ac} \cdot \frac{Ca}{Ba} \cdot \frac{Bc}{Cb}}} = 1.}} & (8)\end{matrix}$

Substituting for Ac in (7) results

$\begin{matrix}{{\frac{Ab}{Ba} \cdot \frac{Ca}{\left( {{{Ks} \cdot {Bc}} + {Ko}} \right)} \cdot \frac{Bc}{Cb}} = {{1{\frac{Ab}{Ba} \cdot \frac{Ca}{\left( {{{Ks} \cdot {Bc}} + {Ko}} \right)} \cdot \frac{Bc}{Cb}}} = 1.}} & (9)\end{matrix}$

And from (8)

$\begin{matrix}{{Ko} = {{{{Bc} \cdot {Ab} \cdot \frac{\left( {{Ac} \cdot {Ba} \cdot {Cb}} \right)}{\left( {{Ca} \cdot {Bc}} \right)}} - {{Ks} \cdot {BbKo}}} = {{{Bc} \cdot {Ab} \cdot \frac{\left( {{Ac} \cdot {Ba} \cdot {Cb}} \right)}{\left( {{Ca} \cdot {Bc}} \right)}} - {{Ks} \cdot {{Bb}.}}}}} & (10)\end{matrix}$

Substituting Ko into (9) gives

${\frac{Ab}{Ba} \cdot \frac{Ca}{{{Ks} \cdot {Bc}} + \left( {{Bc} \cdot {Ab} \cdot \frac{{Ac} \cdot {Ba} \cdot {Cb}}{{Cb} \cdot {Bc}}} \right) - {{Ks} \cdot {Bb}}} \cdot \frac{Bc}{Cb}} = 1.$

This can be solved for Ks, resulting in:

$\begin{matrix}{{{Ks} = {\left( \frac{1}{\left( {{Bc} - {Bb}} \right)} \right) \cdot \left( {\left( \frac{{Ba} \cdot {Cb}}{{Ab} \cdot {Bc} \cdot {Ca}} \right) - \left( \frac{{Ab} \cdot {Ac} \cdot {Ba} \cdot {Cb}}{{Bc} \cdot {Cb}} \right)} \right)}}{{Ks} = {\frac{1}{{Bc} - {Bb}} \cdot {\left( {\frac{{Ba} \cdot {Cb}}{{Ac} \cdot {Ba} \cdot {Cb}} - \frac{{Ab} \cdot {Ac} \cdot {Ba} \cdot {Cb}}{{bc} \cdot {Cb}}} \right).}}}} & (11)\end{matrix}$

From (11) and (10), the two calibration parameters, which have to beapplied to the detected signal from coil 2 b can be determined. Whenthose correction factors are applied, the coil 2 a and 2 b will now readthe correct values for an applied field, not only for the calibrationfield but also for a field from a structure 10 to be detected.

Optionally, in order to verify that the coil array will read the depthcorrectly, an electromagnetic field of known frequency and intensity canbe introduced deliberately. As the depth to a buried service, which canbe considered as a long current carrying conductor, is given by:

${{Depth} = \frac{1}{\left( \frac{A}{B} \right) - 1}},$

this could be verified by applying a known difference in current to eachof the transmitters and repeating the above analysis.

From above calibration and formulas, also the values of d1 and d2 can becalculated, which can e.g. be defined with respect to a central axis ofthe coil windings.

The herein shown formulas are shown exemplary to explain the basicprinciple and to show that it is physically and logically possible toachieve a calibration based on the principle according to the presentinvention. In other embodiments of the present invention, the basicformulas from above can be modified, e.g. to better match the actualphysical conditions. For example the magnetic field formulas can beadapted to an actual coil design, the calibration parameters can bechosen differently, parameter estimation, a least square fit, anonlinear calibration model, etc. can be used to determine thecalibration parameters, etc.

In a specific embodiment of the present invention, the detection coils 2a,2 b,2 c can also be paired with additional dedicated transmissioncoils for the excitation signal, which are aligned in the same plane andmagnetically coaxial to the detection coil, wherein they can beelectrically separated or share one of their ports.

The embodiment of FIG. 3a , FIG. 3b and FIG. 3c can in other word bedescribed as a detection device 1, comprising at least a first 2 a, asecond 2 b and a third 2 c detection coil as well as a configurationunit, which is configured and built in such a way to configure one ofthe at least first 2 a, second 2 b or third 2 c detection coil as atransmitter for a calibration signal by an application of an electricalexcitation signal to the one coil, which calibration signal emanates acalibration field At,Bt,Ct which is sensed by at least two of theremaining of the detection coils to which the excitation signal is notapplied, and calibration parameters for at least two of the coils aredetermined based on this sensed calibration field. In particular each ofthe first, second and third coil is subsequently energized by theexcitation signal in turn and thereof resulting signals are measured bythe remaining of the first, second and third coils, to self calibratethe device by determining calibration parameters for nullifying outdeviations, in particular any offsets and gains.

FIG. 4a , FIG. 4a and FIG. 4a are showing some examples of specificembodiments of a detection device 1,1 a,1 b which can, according to aspecial aspect of the invention, be resized. As it can be seen from thebasic formula, the depth determination for a service 10 is dependent onthe distance 3,3 a,3 b in-between the coils 2 a,2 b,2 c, wherein greaterdistances 3,3 a,3 b are favourable for an accurate determination of thedepth “a”°13 of a service. Therefore, the devices 1 in prior art have tobe rather longish to achieve a reasonable separation 3,3 a,3 b of thecoils 2 a,2 b,2 c. Although favourable for detection, this large size isnot favourable in view of storage and/or transportation of the device 1.According to this special aspect of the invention, the device 1,1 a,1 bis built in an expandable way, e.g. in a telescopic, collapsible,foldable design. Therein, the coils 2 a,2 b,2 c can be brought into afirst detection location, which has a large coil spacing that allowsreasonable depth detection. In the second storage or transportationlocation, the coils locations are brought into small spacing, which isnarrower than the large spacing.

As the separation 3,3 a,3 b of the coils 2 a,2 b,2 c has to be exactlyknown since it influences the determined depth “a”13—such an extensibledesign can suffer from reduced depth accuracy, since the coil locationswith respect to each other might vary from extension to extension.Therefore, in prior art the coils 2 a,2 b,2 c were rigidly fixed withrespect to each other. The spacing in-between the coils 2 a,2 b,2 c wasusually not used for other purposes than for providing stability and wasfor example mostly filled with air. By the herein proposed selfcalibration, this drawback can be overcome, since influences of a slightvarying coil location in the extended position can be determined in thecalibration parameters and therefore compensated—allowing a higherguaranteed detection accuracy for a extendible device 1,1 a,1 b thanwithout.

FIG. 4a shows the first extended detection location of the coils 2 a and2 b in the device 1, in which extended arrangement the detection orcalibration is carried out. FIG. 4b shows an example of a secondcompressed storage location of the coils 2 a and 2 b in which the device1 b can be stored and/or transported. In this example of an embodiment,a folding- or hinge-mechanism is shown, indicated by the arrow 21 b. Inthe example of an embodiment in FIG. 4c a telescopic mechanism is usedfor providing a variability to the coils 2 a,2 b locations in the device1 c, which is indicated by the arrow 21 c. Still, there are also otherknown techniques to provide and implement the mechanical extendibilityaccording to this aspect of the invention. The same principles can alsobe used if more than the two shown coils 2 a,2 b are used, e.g. for thethree coils 2 a,2 b,2 c of the example above or more. The first and/orthe second location can therein be embodied to be blocked during theirrespective usage.

FIG. 5 shows an example of a basic block diagram of an embodiment of thepresent invention that shows a functional flow of the self-calibrationaccording to the invention.

In block 50 the calibration routine according to the present inventionis started. As it is a self-calibration of the device 1, this can bedone independent from external equipment, for example in the fieldbefore the device 1 is used for depth detection.

In block 51, the calibration unit of the device 1 configures one of thedetection coils 2 a,2 b,2 c as transmitter. This comprises block 52,with an applying of an electrical signal source to the one coil, forexample by generating an electrical excitation signal and switching thissignal to the one coil. This one coil then emits a calibration fieldbased on the excitation signal. As shown by block 53, the resultingcalibration field can be detected by the remaining detection coils,which are not configured as a transmitter. Those remaining detectioncoils are configured in ordinary receiving mode, as they are, when anoccluded structure is detected in a detection mode after thecalibration. The values of the detected signals from the calibrationfields are stored.

In block 54, a permutation of the one coil that was used as atransmitter is done towards another coil. In particular the coils aresubsequently configured as transmitter, preferably all of the coilspresent in the device are configured as transmitter one after the other,while the remaining coils are detecting the resulting calibrationfields. This is indicated by the loop 55, which is done for multiple, inparticular all of the coils.

In block 56, the calibration parameters are determined based on thestored values of the received fields during above calibration loop. Thecalibration parameters, which can in a simple linear error modelcomprise an offset and a gain parameter but can also be chosendifferently, are then determined from the stored values, considering theunderlying physical principles and dependencies.

To avoid influences from environmental fields, in a certain embodiment,the excitation signal can be chosen to have special characteristics likea frequency, modulation or coding, according to which it can beidentified and/or distinguished from environmental signals by thecalibration unit. In a special embodiment—where only a correlatedportion of the signals from the at least two detection coils is used todetermine the depth, e.g. the proximity determination is done with afiltering which can only be passed by a portion of signals from coil 2 aand 2 b, which is correlated to each other, wherein an additionaltime/phase shift of those signals due to the coil spacing 3, etc. can beconsidered—such a correlation between the received signals and/or theexcitation signal can also be considered for the calibration accordingto the present invention.

The following special aspect might also be considered as an invention onits own, when seen without the calibration presented above. In additionto the calibration, but also for the calibration, it is also importantfor the performance of the detection device 1 to achieve highsensitivity, high linearity and a good signal to noise ratio.

The traditional method of designing a coil 2 a,2 b,2 c for cabledetection is based on Faradays law of induction. For example, an oftenused Rogowski Coil has an output voltage which is proportional to thenumber of turns “n”. In order to obtain more sensitivity or voltageoutput, it is therefore apparent to simply add more turns of wire ontothe coil. Cable detection coil designs in prior art often have 10′000'sturns or more and are therefore large and heavy. Those many turns areoften separated into multiple channels, so that the wire windings areseparated to reduce the self capacitance of the coil.

A typical known detection coil 2 with its corresponding electronicsdesign 63 implementing a voltage detection principle is shown in FIG. 6.The detected magnetic B field 61 generates an output voltage Vout at theterminal 62 of the detection coil 2. For example, a common useddetection coil L2 2 can have about 900 turns, an average windingdiameter of about 70 mm, with a series resistance of about 470 Ohm, aself capacitance of about 5 nF and an inductance of about 140 mH.Analyzing a thereto corresponding electrical equivalent circuit resultsin an expression for the sensitivity of the coil on its own inVolts/Tesla of

V/B=j·2·π² ·f·n·a ².  (20)

With f=frequency, n=number of turns, a=radius of a Rogowski coil,B=magnetic flux, V=output voltage.

In formula (20), the sensitivity is proportional to f, n, and a² and notdependent on the inductance L.

In a practical embodiment, the proportionality to the winding count nand to the enclosed coil area a², fits quite well, but for the frequencyresponse also the evaluation circuit 63 connected to the coil 2 at port62 and its input impedance R_(in) has to be considered. This results inan expression for the sensitivity at high frequency that becomes

V/B=π·n·a ² ·Rin/L.  (21)

This expression (21) for the sensitivity is now independent offrequency, but in a practical application there is a roll off at higherfrequencies due to the self capacitance of the coil, which is notincluded in above formula for simplicity.

FIG. 7 shows an example of a typical baud-plot, with sensitivity 67 andphase 66 over frequency, of a commonly used circuit like the one of FIG.6. It reveals a nonlinear sensitivity with a resonance at about 5 kHz,but neither mains-frequencies, artificially induced detectionfrequencies nor typically naturally emitted Radio frequencies, which areto be detected by a detection device benefit from this resonance.

FIG. 8 shows a typical example of a corresponding noise plot overfrequency of a commonly used circuit like the one of FIG. 6. The noisecurve 68 has its highest noise levels in the low frequency ranges, whichhave to be sensed for mains detection, and a noise peak in the range ofthe resonance frequency from above.

Prior to or without the following aspect of the present invention, thepractical problems encountered with designing a detection coil 2 was toachieve many turns with low self capacitance, which limits the highfrequency operation. As a solution, various methods have been devisedlike wave winding and segregation of the windings into multiplesegments, all of those consequently making the volume of the coil 2 evenlarger.

It can be seen that above-mentioned known and used approach fordesigning detection coil 2 evaluation circuitries 63, has its drawbacks,in particular in view of nonlinearity, sensitivity and noise, but alsoin the large coils with many windings which are required. In view of thecalibration according to the invention, in particular the nonlinearityover frequency, but also the noise and sensitivity can hinder anaccurate calibration. For example, in view of the nonlinearity it canlikely be required to calibrate each of the detection frequency bandsseparately.

According to this aspect of the invention, an alternative design for theabove described detection circuit for a cable detection device isproposed. Compared to prior art, the approach of this aspect of theinvention is more linear in its output, not dependent on the number ofturns and also has higher sensitivity at low frequencies, as explainedin detail below. In particular, the calibration according to the presentinvention will benefit there from, but it brings also advantages on itsown. Without the calibration aspect, it might be considered as astandalone invention in the field of the art of cable detection.

FIG. 9 shows the basics of a current detection model according to thisaspect of the invention. As in FIG. 6 before, B represents the magneticfield 61 penetrating the coil 2, Vout 71 is the output voltage of thereception circuitry and I is the current flowing through the coil 2. Thecoil 2, which comprises one or more loops of an electrical wireconductor, can as before be modelled by its inductance L, its seriesresistance R and its capacity C. The amplifier according to this aspectof the invention is a current amplification stage, with an operationalamplifier 70 and its current input mode feedback network R_(f). R_(f)symbolized a feedback network used with the amplifier according to theaspect of the invention.

Thereof, a sensitivity expression can be derived as:

$\begin{matrix}{{\frac{V}{B} = {- {\frac{n \cdot \pi \cdot a^{2} \cdot {Rf}}{L}.}}},} & (22)\end{matrix}$

with R_(f)=feedback impedance, V=output voltage, L=coil-inductance,B=magnetic field, n=number of turns, a=radius of the coil.

It can already be noted that above formula (22) suggest that thesensitivity is now independent of frequency. Considering the inductanceL, a coil of e.g. of rectangular cross section can be defined by theequation:

L=P ₀ ·a·n ²,  (23)

where “a” is the mean coil-radius, “n” the number of turns and P₀ thecoil-coefficient which can be found in textbooks. There are also otherformulas for a numerical determination of a coils inductance known, butin rough approach they are also approximately proportional to a and thesquare of n.

Substituting above formula (23) in formula (22) and considering theseries resistance of the coil 2 with its additional parametersρ=resistivity of the coil winding material and s=cross sectional area ofthe conductor of the winding, and by considering Ohms law gives anexpression for the sensitivity of:

$\frac{Vout}{B} = {\frac{j \cdot \pi \cdot f \cdot a \cdot s \cdot {Rf}}{\rho}.}$

The above shown expression for the sensitivity is not depend on thenumber n of turns and is inversely proportional to the resistance of thewire. According to this aspect of the present invention therefore, a lownumber of turns of a thick conductor is favoured over the high windingcount used in prior art approaches.

According to the theoretical formula above, a single turn of thick,highly conductive wire would be the preferred option, as thereby a highsensitivity at the output can be obtained, which is not frequencydependant. Nevertheless, when considering non ideal components, inparticular amplifiers 70 the principle of low winding count and a largecross-section, highly conductive winding material remains, but has to beoptimized in to the used components, so that a winding count greaterthan one, but still low, say below a few hundred turns might bepractically implemented as optimal compromise which also depends on theselected type of OpAmp 70 and can e.g. be determined by simulation. Forhigh sensitivity, there is also a low noise design of the components andits setup required, in order to measure the desired weak electromagneticfields, in particular in the desired frequency range of about 50 Hz to250 kHz that is used in cable detection. For the overall performance notonly the sensitivity, but also the achievable SNR has to be considered.If just a single turn of large diameter wire is used, for maximumsensitivity, then the resistance is essentially a short circuit to theoperational amplifier. Therefore, the offset and current noisespecification of the amplifier has to be carefully considered orotherwise it might overlay the improved sensitivity by a higher noisefloor, hindering the detection of the desired signal.

FIG. 10 shows an example of an embodiment, with the detection coil 2,which can—as discussed at top—be configured as a transmitter by thecalibration unit, when the switch 72 contacts the excitation signalsource 73, or be configured in ordinary receiving mode, when the switch72 contacts the amplification stage according to the aspect of inventiondiscussed right before. The values of the used parts are onlyillustrative and not limiting. In another here not shown embodiment, theamplification stage can be directly connected to the coil, whereby anapplied excitation signal from 73 can also be detected by the samecircuit which otherwise detects the received signals.

FIG. 11 shows another example of another embodiment with characteristicsaccording to the aspect of the present invention, wherein the inputstage connected to the coil comprises a JFET J1 in the first stage, fedback with an OpAmp U1. The bootstrapping circuit 75 is shown as anexemplary embodiment and can also be realized differently. In additionto the detection circuit for the coil 2 a of the certain aspect of theinvention, there is also a coil 2 b of the detection device shown, whichis configured for transmission by the calibration unit. The signalsource 73 applies an excitation signal to the coil 2 b, which emits acalibration field B which couples to the detection coil 2 b that islocated with a spacing from coil 2 a, to achieve the calibration asdiscussed before.

FIG. 12 shows a baud-blot with phase 76 and amplitude 77 of detectionaccording to the present aspect of the invention, which is obviouslymuch more linear than the one of FIG. 7, in particular when consideringthe change in scales. Also, the noise curve 78 shown in FIG. 13 hasimproved compared to the one of FIG. 8.

Some examples from experimental results, which were evaluated with adifferent number of turns and different wire diameters are exemplaryshown in FIG. 14. The diagrams are showing the output voltages Vout asindicated in FIG. 9, FIG. 10 or FIG. 11 over a frequency range in kHz,which is resulting when the coil 2 is provided with an electromagneticfield of defined strength at this frequency.

As an example of typical dimensional ranges according to the certainaspect of the invention, a coil-embodiment with about 20 turns of wire,wound to a coil of about 90 mm diameter can be considered. Theembodiments are made from wire diameters of 0.71 mm, 0.315 mm, althoughother diameters might be used as well. Apparently, it is not required touse a round cross section material for the winding, e.g. also squarerods or foil strips can be used to form one or more windings accordingto the invention. The winding should consist of a good conductor,preferably copper, but also e.g. aluminium, gold or silver. The diagramshows that a larger diameter wire (generally a higher cross-sectionarea) achieves more sensitivity at lower frequencies than smaller ones.In comparison, the prior art approach with a high turn, voltageamplified coil is shown in comparison. In particular in view of thedesired frequency ranges being most relevant for the given buriedservice detection application, the improved sensitivity in those desiredbands in the low frequency range of 50 to 200 Hz can be seen.

In an example, which is discussed based on the exemplaryworking-embodiment shown in FIG. 15, the desired frequency band to becovered will be from about 50 Hz to about 250 kHz. Also, other factorslike low power consumption, lightweight, decent temperature ranges forfield applications, and small dimensions are factors that are to beconsidered. The coil 2 a,2 b is designed with exemplary dimensions,which are reasonable for a handheld cable location tool and preferablysmaller than prior art devices while having the same sensitivity. Forexample, the coils can be set to about a square of 80 mm×80 mm—whichequals to a mean radius of about 90 mm. A coil area in that order hasproven to provide sufficient sensitivity required to achieve a 5 mdetection distance from a buried service 10 for the present detectiondevices 1, which roughly equals to 4E-9 Tesla, wherefore a targetedsensitivity for magnetic fields can be around E-12 Tesla.

As a special aspect of this embodiment of the present invention, thecoils 2 a,2 b are embodied by tracks on a PCB 80 a,80 b, which printedcircuit board can also comprise at least part of the electronics of thedetection device 1, in particular a current amplification stage 81according to the special aspect of the invention, which actually allowsto achieve reasonable detection performance with the low winding countsthat can be embodied in a PCB, which will in general be about or lessthan 100 turns. If desired, the copper layer on the PCB can be chosenrather thick, to also achieve a large cross section area of the winding.

There are two PCBs 80 a and 80 b, each with a detection coil 2A resp. 2b shown, with their coil axis 82 spaced by the distance 3, but alsoother coil configurations, like e.g. a three coil arrangement as in FIG.3a can be realized and/or the expandability of FIG. 4b or FIG. 4c can becombined. By the shown design, a lightweight detection device 1 of smallsize can be built. The PCB coils allow an even smaller, lighter andefficient design and gain noise immunity when the first amplificationstage 81 is located close to the coil. Nevertheless, also wire woundcoils could be used in another embodiment, in particular with a lowcount of turns of a thick wire—as described above.

The shown device 1 also comprises a calibration unit which is built insuch a way that for calibration each one of the coils 2 a,2 b within thedevice 1 is subsequently configured as transmitters by an application ofan excitation signal. The thereby transmitted calibration field ispicked up by the remaining coils and a self-calibration of the device isdetermined as calibration parameters for the coils 2 a,2 b, based on thepicked up signals. The calibration also gains additional advantages ofthe high linearity of the current amplifier aspect of the detection,whereby the accuracy and sensitivity of the device 1 can be furtherimproved. By the calibration parameters, not only the coils 2 a,2 b andtheir spacing 3, but also the evaluation circuit, e.g. comprising thecurrent amplifier, possible additional amplifiers and/or filters, and anAnalog to Digital converter will be comprised in the calibration.

A skilled person is aware of the fact that details, which are here shownand explained with respect to different embodiments, can also becombined with details from other embodiments and in other permutationsin the sense of the invention.

What is claimed is:
 1. A mobile detection device for an evaluation of adistance/depth value from the device to an occluded AC-current carryingstructure according to its emanated magnetic field, with a plurality ofdetection coils, each comprising at least one winding of an electricallyconductive material, which are arranged at a fixed location with aspacing with respect to one another; and an electronic signal evaluationunit for detecting the structure according to an electrical signalinduced in the plurality of detection coils by the magnetic field andevaluating the distance value according to a difference of theelectrical signal in-between at least two of the plurality of detectioncoils; wherein the the mobile detection device comprises: a calibrationunit built to configure one of the plurality of detection coils astransmitter for a calibration field by applying an electrical excitationsignal, which calibration field is detected by the remaining of theplurality detection coils and wherein calibration parameters for theplurality of detection coils are determined by the calibration unitbased on the detected calibration field.
 2. The mobile detection deviceaccording to claim 1, wherein the calibration parameters comprise atleast one of a calibration scaling factor and a calibration offset. 3.The mobile detection device according to claim 1, wherein: the mobiledetection device comprises at least a first and a second of theplurality of detection coils, and the calibration unit is built tosuccessively configure one of the detection coils as transmitter for thecalibration field, while detecting the calibration field with theremaining detection coil.
 4. The mobile detection device according toclaim 1, wherein: the mobile detection device comprises at least afirst, a second, and a third detection coil of the plurality ofdetection coils, and the calibration unit is built to successively applythe electrical excitation signal to one of the at least first, second orthird detection coil to transmit the calibration field, whichcalibration field is detected by the two remaining of the detectioncoils to which the excitation signal is not applied, wherein thecalibration parameters for at least two of the coils are determinedbased on the detected calibration field and wherein the calibrationparameters also comprise a spacing parameter of the detection coils forat least two of the detection coils.
 5. The mobile detection deviceaccording to claim 1, wherein: the mobile detection device comprisesonly a first, a second, and a third detection coil of the plurality ofdetection coils, and the calibration unit is built to successively applythe electrical excitation signal to one of the at least first, second orthird detection coil to transmit the calibration field, whichcalibration field is detected by the two remaining of the detectioncoils to which the excitation signal is not applied, wherein thecalibration parameters for at least two of the coils are determinedbased on the detected calibration field and wherein the calibrationparameters also comprise a spacing parameter of the detection coils forat least two of the detection coils.
 6. The mobile detection deviceaccording to claim 1, wherein: the electrical excitation signal isgenerated by a current signal generator, preferably by one single signalgenerator whose output is selectively switchable to one of the detectioncoils, and wherein the excitation signal is a sine wave of knownfrequency, preferably having a variable or modulateable amplitude and/orfrequency.
 7. The mobile detection device according to claim 6, whereinthe calibration signal is modulated or coded in such a way to make ituniquely distinguishable from other environmental signals and noise. 8.The mobile detection device according to claim 1, wherein: theelectronic signal evaluation unit comprises an amplifier circuitembodied as a current sensing amplifier as a first amplification stagewhich is connected to the detection coil.
 9. The mobile detection deviceaccording to claim 1, wherein: the electronic signal evaluation unitcomprises an amplifier circuit embodied as a transimpedance amplifier asa first amplification stage which is connected to the detection coil.10. The mobile detection device according to claim 1, wherein: theelectronic signal evaluation unit comprises an amplifier circuitembodied as a current sensing amplifier as a first amplification stagewhich is connected to the detection coil having a low input impedance,wherein the amplifier circuit is built to be tuned to have asubstantially linear output over frequency in combination with the coil.11. The mobile detection device according to claim 1, wherein: theelectronic signal evaluation unit comprises an amplifier circuitembodied as a current sensing amplifier as a first amplification stagewhich is connected to the detection coil having a low input impedance,wherein the amplifier circuit is built to be tuned to have asubstantially linear output over frequency in combination with the coil.12. The mobile detection device according to claim 1, wherein: thedetection coils each has a winding count of 1 to 500 turns of aconductor with a cross-section of at least 0.1 mm², wherein the windingof the detection coil encloses an area of more than 100 cm² and below0.5 m², with an approximately rectangular or circular cross-section. 13.The mobile detection device according to claim 1, wherein: the currentsensing amplifier comprises an operational amplifier OpAmp with afeedback network in a current to voltage configuration, wherein thenegative OpAmp input is connected to one end of the coil; the other endof the coil is connected to the positive OpAmp input; and the OpAmpoutput is fed back to the negative OpAmp input by a first impedance. 14.The mobile detection device according to claim 1, wherein: the currentsensing amplifier is embodied with an input stage comprising a JFET in afeedback loop of a first operational amplifier stage in a current tovoltage configuration, wherein the JFETs-Gate is connected to thedetection coil and the feedback network, preferably with an activeoperation point setting comprising a second operational amplifier forbiasing the first operational amplifier stage.
 15. The mobile detectiondevice according to claim 1, wherein: the output impedance of thedetection coil is above the input impedance of the first amplificationstage which the detection coil is connected to.
 16. The mobile detectiondevice according to claim 1, wherein: the detection coils are embodiedas tracks on a PCB, wherein the PCB also comprises the current sensingamplifier.
 17. The mobile detection device according to claim 1,wherein: the detection coil has a dedicated transmitter winding portionfor application of the excitation signal and a dedicated receiverwinding portion for detecting magnetic fields, preferably wherein bothwinding portions are magnetically coaxial to each other, wherein the twoportions are on a common rigid carrier.
 18. A calibration method for amobile detection device which comprises a plurality of detection coilsfor an evaluation of a distance value from the detection device to anoccluded AC-current carrying structure according to the structuresemanated magnetic field, the method comprising: applying an electricalexcitation signal to one of the plurality of detection coils, which thenacts as a magnetic field transmitter for a calibration field; receivingand evaluating the calibration field by the remaining detection coil ofthe plurality of detection coils, which is done multiple times with apermutation of the plurality of detection coils usage for transmittingor receiving; and determining calibration parameters for at least two ofthe plurality of detection coils is done by a calibration unit, based ona set of the evaluated calibration fields from each permutation.
 19. Themethod according to claim 13, wherein: the calibration parameterscomprise an offset and gain calibration parameter.
 20. The methodaccording to preceding claim 13, wherein: the detection device comprisesat least a first, a second and a third detection coil of the pluralityof detection coils, and the calibration unit is successively applying anelectrical excitation signal to one of the at least first, second orthird detection coils for transmitting the calibration field, and adetecting of the calibration field by the remaining of the detectioncoils to which the excitation signal is not applied and storing itsvalues, wherein the determining of the calibration parameters comprise again parameter and an offset parameter for each detection coil,preferably also a spacing parameter of the detection coils.
 21. Themethod according to preceding claim 13, wherein: the detection devicecomprises only a first, a second and a third detection coil of theplurality of detection coils, and the calibration unit is successivelyapplying an electrical excitation signal to one of the at least first,second or third detection coils for transmitting the calibration field,and a detecting of the calibration field by the remaining of thedetection coils to which the excitation signal is not applied andstoring its values, wherein the determining of the calibrationparameters comprise a gain parameter and an offset parameter for eachdetection coil, preferably also a spacing parameter of the detectioncoils.
 22. The method according to claim 13, wherein: the receiving andevaluating with the detection coils comprises an amplifying of thedetection coils output in current mode by a current mode amplifier whichis providing low input impedance to the detection coil, wherein thewinding count is of the coil is below
 100. 23. A non-transitory computerprogram product comprising program code stored on a machine-readablemedium, or computer-data-signal embodied as an electromagnetic wave, fora calibration of a detection device, built for executing: an applying ofan excitation signal to one detection coil of the detection device foremanating a calibration field, a receiving of the calibration field byat least one other coil of the detection device and storing a fieldvalue, wherein the applying and receiving is done alternately for thedetection coils of the detection device, preferably to cover allpossible permutations, and a calculating of calibration parameters forthe detection device based on the field values.